Mounting of optical components

ABSTRACT

An optical element ( 3 ) is mechanically retained within a slot of ( 2 ) in a rigid substrate ( 1 ), e.g. of a ceramic or crystalline material, by contact with the sides of the slot. Where the difference between a dimension of the optical element and the slot width at ambient temperature is a very small and positive, it may be removed by heating the substrate and/or cooling the element (differential thermal expansion) to allow insertion of the element. Then the temperature difference is removed so that the element is mechanically retained under compression by the slot sides. Alternatively, where the element may be directly inserted so that it is in contact with or immediately adjacent the slot wall, the material of the substrate and/or the optical element is the oxidized in the region of contact so as to increase the dimension of the oxidized material and mechanically retain the element. Either technique is applicable to parallel sides slots, but the element may be additionally or alternatively mechanically retained by shaping the edges of the slot. The fibre ( 3 ) shown may be butted against a second fibre, or coupled to a guide or other element within the substrate.

[0001] The present invention relates to the mounting of an opticalelement to a substrate, and particularly but not exclusively to thejoining of an optic fibre to a substrate and to coupling of an opticfibre to a further optical element located in or on a substrate.

[0002] A number of techniques are known for coupling an optic fibre toan on-chip optical waveguide or chip facet. The most common approach isto align the fibre and to secure it in place by the use of a UV-curingepoxy adhesive. Alignment is achieved merely by butting the end of thefibre to the waveguide/chip facet, or, for better location, by securingthe fibre with adhesive within a V-groove in the chip. One of the majorproblems associated with the use of epoxy adhesives is that they candegrade over time due to temperature cycling and ageing effects, causingmisalignment between fibre and waveguide/chip facet.

[0003] In “Silicon Nitride Micro-Clips for the Kinematic Location ofOptic Fibres in Silicon V-shaped grooves”, R M Bostock et al, JMicromech & Microeng, 8 (1998), page 343, there is reported a techniquein which flexible clips are surface-machined out of silicon nitride andare used to hold a fibre in place in a V-groove. However, there aredisadvantages in that micro-manipulation is necessary to insert thefibre beneath the clips, and multiple process steps are required to formthe clips in the first place.

[0004] The prior art techniques mentioned above rely on manual orsemi-automated alignment and are time-consuming.

[0005] The present invention provides a mechanical approach in whichself-alignment is possible, thereby facilitating the production of areliable coupling with repeatability, and a more automated approach. Theresulting product is resistant to ageing and cycling over temperaturestypically experienced in field use of described under the appropriatemilitary standards, and ideally is also mechanically strong.

[0006] In a first aspect, the present invention provides an opticalarrangement comprising a first rigid substrate having a slot and a firstoptical element mounted within the slot, said element being mechanicallyretained by contact with the sides of the slot.

[0007] International Patent Application No. WO 96/38752 (WhitakerCorporation), U.S. Pat. No. 5,566,269 (Whitaker Corporation) and U.S.Pat. No. 5,359,687 (McFarland) each disclose constructions formechanically holding optic fibres with strain relief, but that strainrelief is afforded by the use of plastics in the constructions which bytheir very nature afford a degree of flexibility. U.S. Pat. No.4,702,547 (Enochs) discloses a method of attaching an optical fibre tothe surface of a substrate using a channelled retaining member which issoldered over the fibre, which itself is provided with an external goldlayer. Retention is therefore not by mechanical contact with the side ofa slot in a substrate, or even by mechanical with the sides of theretaining member.

[0008] The rigid substrate may be of a ceramic material, such as glassor silicon, or a crystalline material, such as silicon.

[0009] The mechanical retention is preferably such that the element isnon-releasably retained, i.e. removal thereof from the slot isaccomplished only with very great difficulty or not at all withoutdestroying the element and/or substrate.

[0010] If the slot has substantially parallel sides, the element may beretained under compression by the sides of the slot. Alternatively, oradditionally, the element may be mechanically retained by the shaping ofthe sides of the slot, which are preferably parallel, but notnecessarily so.

[0011] Mechanical retention by compression may be achieved by the use ofdifferential thermal expansion, for example by cooling the elementand/or heating the substrate, placing the element within the slot andallowing the temperature difference between the element and substrate todisappear.

[0012] Shaping of the sides of the slot may be achieved by a chemicalreaction such as oxidation of the first element and/or the firstsubstrate in the region of contact, for example where the first elementis of silica, e.g. a fibre, and the first substrate is of silicon orother suitable oxidisable material, or where the first element is ofsilicon, and the first substrate is of silicon or silica. The conversionof the silicon to silica is accompanied by an increase in the dimensionof the material resulting in the slot becoming narrower, and/or theelement becoming thicker, and/or the wall of the slot growing round theelement. It will be understood that the first two of these effectsrelate to increasing the compressive force exerted by the slot walls onthen element, whereas the third effect limits movement of the elementvertically within the slot. Thermal oxidation is mentioned in thedescription of the embodiments, and it could be used in conjunction withthe differential thermal expansion effect. However, it is also envisagedthat a relatively cool oxidation or other wall shaping process may beemployed per se.

[0013] The first element may be mounted so that it is in substantialoptical alignment with an optical component within the substrate, forexample in substantial abutment with the optical component. This opticalcomponent, for example a waveguide, may itself be formed within thefirst substrate. Alternatively it may be a second optical element suchas an optic fibre which is mechanically retained by the sides of theslot, e.g. under compression and/or by wall shaping.

[0014] The slot in the first substrate may commence at one edge of thefirst substrate. It may extend between opposed edges of the firstsubstrate.

[0015] The first element may be an optic fibre. The first substrate maybe of a semiconductor material such as silicon.

[0016] The first element and/or a second element may be further securedto the first substrate by any means known per se, including the use ofan adhesive.

[0017] Where the first element is an optic fibre, the first substratemay be mounted to a second substrate. In such a case, a surface of saidsecond substrate may include an optical component mounted or formedthereon, the first substrate being mounted to the said surface of thesecond substrate with the optic fibre in alignment with said opticalcomponent.

[0018] Alternatively, a surface of the second substrate may include anoptical component recessed or formed therein, with the first substratebeing mounted in a depression in the said surface of the secondsubstrate with the optic fibre in alignment with said optical component.

[0019] Preferably, the first and second substrate have co-operatingprojections and holes or slots for relative alignment thereof. Forsecuring the substrates together, at least one projection may be formedto be a slightly oversize fit in its hole or slot, such that it may becooled relative thereto for insertion, and is gripped thereby ontemperature equalisation. It will be appreciated that the projection andhole may or may not have corresponding shapes, so that the gripping mayoccur in one or both dimensions, and to avoid excessive stress it ispreferred that the gripping is effected in one dimension only (forexample by use of an elliptical peg and a circular hole or vice versa,or the use of a slot to accommodate the projection).

[0020] By forming the projection(s) to be so gripped in one or bothdimensions, and/or by forming projection(s) to have an accurate fit inone or both dimensions, the location of the first substrate relative tothe second may be precisely controlled in the corresponding dimension ordimensions. If only one projection is oversize or an accurate fit, andit is circular, it is possible to rotate one substrate relative to theother, so for full location a second oversize or accurately fittingprojection is required.

[0021] Alternatively the single projection and its hole may have anon-circular section to prevent rotation. Other projections, if any, inthese arrangements may have an accurate or oversize fitting in theirholes or slots, or some or all may have a slack or loose fit.

[0022] Alternatively, all the projections may be sized and shapedrelative to the holes (or slots may be used) so that there is latitudefor adjustment of alignment of the first substrate along one or bothdimensions of the second substrate surface. In this case a differentmeans is preferably provided for securing the substrates together.

[0023] Any means known per se, including the use of adhesive may be usedfor this purpose, but where the first substrate within a depression inthe second substrate, preferably the depression has vertical opposedwalls between which said first substrate is gripped mechanically undercompression. This technique may be used per se, or in addition to theprovision of a gripped projection(s).

[0024] Where the first element is an optic fibre, an end thereof may liesubstantially flush with an edge of the second substrate. If the fibrehas a lateral portion projecting above the surface of the firstsubstrate, the second substrate may be recessed to accommodate theprojecting portion. The invention also provides a method of mounting anoptical element in a substrate by forming a parallel sided slot in thesubstrate, the difference between a dimension of the optical element andthe slot width at ambient temperature being a very small positiveamount, producing a positive temperature differential between thesubstrate in the region of the slot and the optical element such thatthe said dimension is now no more than the slot width, inserting theoptical element therein and allowing the said temperature differentialto diminish. The temperature differential may be produced by the heatingof the substrate, or the cooling of the optical element, or both.

[0025] The invention further provides a method of mounting an opticalelement in a substrate by forming a slot in the substrate, e.g. aparallel sided slot, inserting the optical element therein so that it isin contact with or immediately adjacent the walls of the slot, andthereafter oxidising the material one or both of the first substrate andthe optical element in the region of contact whereby to increase thedimension of the oxidised material so to mechanically retain the elementwithin the slot. It will be appreciated that the tolerance permissiblewith respect to initial contact with the slot walls will depend on howgreat a dimensional increase can be obtained for successful mechanicalretention.

[0026] The optical element may be an optic fibre, so that the saiddimension is the diameter of the fibre for a circular cross-section, ora width thereof for a non-circular cross-section (this could be eitherwidth for a rectangular section, but will normally be the smallerdiameter for an elliptical section fibre). In such a case the slot depthneeds to be at least equal to the radius of the fibre.

[0027] The slot may be formed with a depth less than the diameter of thefibre(s) so that a portion thereof projects above the surface of thefirst substrate, or a depth equal to or greater than the diameter of thefibre(s).

[0028] Wherein the substrate includes or is provided with an opticalcomponent with an input surface or aperture, the inserting step maycomprise butting an end of the optic fibre against said aperture. Insuch a case, the slot forming step may be controlled so that the fibrecontacts the bottom of the slot when in alignment with said aperture.The component provided on the substrate may be formed integrally withinthe substrate, adjacent or spaced below its surface, or a separatecomponent which is mounted within the substrate or within a section ofthe slot.

[0029] In the latter case, the method may include the step of mountingthe component (for example another optic fibre) within the section byproducing a positive temperature differential between the slot sectionand the component, inserting the component, and allowing thedifferential to disappear. The mounting of the fibre and the separatecomponent to the substrate may be performed sequentially (in eitherorder) or simultaneously.

[0030] Once the optic fibre has been mounted to the first substrate, thelatter may in turn be mounted the first substrate to a second substratewith the fibre in optical alignment with (and preferably abutting) anoptical component on or in the second substrate. If the fibre projectsabove the first substrate surface, preferably but not necessarily agroove is formed in the substrate to house the projecting portion of thefibre.

[0031] The invention also encompasses a method of coupling two opticfibres comprising the steps of forming a parallel vertical sided slot inthe surface of a first substrate, the difference between the diameter ofthe fibres and the slot width at ambient temperature being a very smallpositive amount, producing a positive temperature differential betweenthe substrate in the region of the slot and the optic fibres such thatthe fibre diameter is now no more than the slot width, inserting theoptic fibres therein with their ends butting, and allowing the saidtemperature differential to disappear.

[0032] The invention extends to constructions in which the firstsubstrate is mounted to a second substrate, and to constructions inwhich one or more further fibres or other components are mounted to thesame substrate, and to associated methods of forming such constructions.

[0033] Further features and advantages of the invention will becomeclear upon a consideration of the appended claims, to which the readeris referred, and upon a reading of the following description ofembodiments of the invention made with reference to the accompanyingdrawings, in which:

[0034]FIGS. 1a to 1 d show plan side and end cross sectional schematicviews of an optic fibre mounted in a substrate according to a firstembodiment of the invention, with FIG. 1d being an enlarged detail ofFIG. 1c;

[0035]FIG. 2 shows a schematic plan view of a second embodiment;

[0036]FIG. 3a shows a perspective schematic view of a third embodiment;

[0037]FIGS. 3b and 3 c show schematic cross-sectional plan and sideviews of the third embodiment;

[0038]FIGS. 4 and 5 show end cross sectional schematic views of fourthand fifth embodiments of the invention:

[0039]FIG. 6 shows a construction similar to that of FIG. 3b, butwherein a chip edge lies flush with a substrate edge; and

[0040]FIG. 7 shows an end cross sectional schematic view of an opticfibre mounted in a substrate and retained therein by wall shaping.

[0041] Where appropriate like reference numerals are used to denote thesame feature in the different embodiments.

[0042] In FIG. 1 a parallel-sided vertical slot 2 is dry etched in asilicon substrate 1, the slot being 124.8 to 124.9 microns wide and 70microns deep. A silica fibre 3 of 125 micron diameter is cooled inliquid helium or liquid nitrogen to cause its diameter to contract to124.65 or 124.75 microns respectively, each of which is less than theslot width. The fibre is inserted into the slot until it touches thebottom 4 of the slot, by which time its diameter 6 is laterally adjacentthe sides 5 of the slot below the chip surface 7, and the temperaturedifferential between the assembled substrate and fibre is then removed,e.g. by allowed the entire assembly to come to ambient temperature.During this process the fibre diameter increases until the fibre isfirmly gripped between the slot sides 5 with a force which is a functionof the precise relative dimensions of the separate fibre and slot.Optionally, the silica fibre 2 is then bonded to the silicon slot walls5 by thermal oxidation to make the assembly even more rugged.

[0043] In this embodiment, the slot terminates at the end face 10 of awaveguide 8 formed just below the surface 7 of the silicon substrate.Thus by firmly butting the end of the optic fibre against the end faceof the waveguide as the fibre is allowed to come to the temperature ofthe substrate an efficient, stable and strong optical coupling betweenthe fibre core 9 and waveguide is obtained. Accurate alignment of thecore 9 relative to the guide 8 in the vertical direction is controlledby suitable adjustment of the slot depth. Although as shown, the slotdepth is such that a portion of the fibre projects above the substratesurface, the slot can be made deeper to accommodate the fibre flush withor below the substrate surface, and in part this will depend of thelocation of the guide 3 and the fibre diameter.

[0044] Furthermore, as shown the slot 2 extends to one edge of thesubstrate. It should be understood that this is not always necessary,particularly with a larger substrate, where the slot may start and endwithin the boundaries of the surface 7, and the fibre 3 may be flexed toenter the slot.

[0045] The second embodiment shown in FIG. 2 is somewhat similar to thefirst. However, the substrate contains no waveguide, and the slot ismodified to extend from one end 12 of the substrate to the opposed end12. The optic fibre and a second fibre 3′ are cooled so that theirdiameters are reduced to slightly below the slot width, and they areboth placed in the slot so that their ends 13 abut around halfway alongthe substrate. The fibres are then allowed to warm to ambienttemperature so that they are gripped by the slot walls. With the fibresinserted so that they touch the bottom surface of the slot, they arefirmly located in all three dimensions and their cores are accuratelyaligned and in contact at their ends for efficient coupling. Again,either or both ends of the slot may terminate within the boundaries ofsurface 7 if the fibres are suitably flexed into position.

[0046] Alternatively, a first fibre 3 is mounted within the slot byrelative cooling and warming as before, so that it terminates within theslot, and subsequently the second fibre 3′ is mounted within the slot byrelative cooling and warming to terminate in abutment and alignment withthe first fibre. This process only requires two components to be joinedat any stage, and either component may be inserted into the slot first.

[0047] In a modification of FIG. 2 (not shown) the slot comprises twosections of different appropriate cross-sectional dimensions, in whichcan be mounted first and second differently sized optical fibres, eithersimultaneously or sequentially, as described above in respect of FIG. 2.

[0048] In FIG. 3 a smaller substrate or “chip” 14 is used for mountingan optic fibre 15, and is itself mounted on a larger substrate 16. As inFIG. 2, the chip 14 is provided with a vertical sided slot 17 whichextends between two opposed faces 20, 21 of the chip, and the slot 17 isdimensioned so that the fibre 7 is mounted and gripped therein bycooling and subsequent warming, and so that the fibre end 22 is flushwith the face 21, or extends therefrom. In the latter case, theprojecting end of the fibre may be used as it is, particularly of theprojection is minor, but preferably the fibre is subsequently treated,for example by grinding/polishing, so that it lies flush with the chipface.

[0049] The bottom face of the chip 10 is provided, e.g. by etching, withoptional locating pegs 18 that are complementary to holes 19 in thesubstrate 8, to provide for accurate alignment of the chip 14 onsubstrate 16 as the chip is moved in the direction of the arrow A.

[0050] On the top surface 26 of the substrate 16 is formed an opticaldevice such as a waveguiding layer 24 above an intermediate layer 25,with its entrance aperture 27 located so that the fibre end 22 abuts itwith its core 23 aligned with the guide when the chip 14 is in position.The chip may then be secured on the substrate by any known means, suchas an adhesive, e.g. epoxy, and/or by any of the differential thermalexpansion techniques described in more detail below.

[0051] The pins 18 may be an accurate fit in the holes 19, to facilitatealignment. Alternatively, there may be some play in one or bothdimensions to permit a final adjustment for optical alignment prior tosecuring the chip. In particular, there may be play in the directionalong the fibre propagation axis, while the fit in the other directionis accurate, so that the best abutment may be secured between the fibre23 and the guide 24, while maintaining the alignment laterally of thefibre direction. Vertical alignment is determined by contact between thefibre, which as shown projects beyond the chip surface, and thesubstrate surface 26. However, if desired, the chip could be mounted sothere is no fibre-surface contact if that provides the best opticalalignment. Nevertheless, it should be noted that in either of theseoptions the bottom surface of the chip is spaced from the surface 26,and these arrangements could lead to excessive leverage being applied tothe pins, for example during use or manipulation.

[0052] Therefore in a modification the substrate surface 26 is providedwith a recess 28 to accommodate the projecting fibre portion as shownschematically in FIG. 4. Although the recess 28 shown as oversized, itwill be appreciated that its wall could contact part or all of theprojecting fibre portion, depending on its shape and dimensions. Therecess provides the additional advantage that a close contact may beprovided between the bottom of the chip 14 and the surface 26, and thelatter can be of use if it is desired to further secure the chip 14 andsubstrate 16 together as by adhesive and/or thermal oxidation.

[0053] It will also be appreciated that the pegs and holes may beinterchanged, and accordingly FIG. 4 shows pegs 29 on substrate 16projecting into complementary holes in the chip 14. Depending on how thechip 14 is secured to the substrate it may even be possible to have pegson each of the chip and substrate for locating in complementary holes inthe other.

[0054] However, in a further modification of the “chip” arrangement, thepegs are gripped in the holes by relative thermal expansion, and in sucha case all the pegs need to be on either the substrate or the chip. Inthis modification, the width of the holes is slightly less than thewidth of the pegs in at least one dimension, and the part bearing thepegs is cooled so that the pegs can be inserted, whereupon on thermalequilibration the pegs are tightly gripped to hold the chip 14 in adesired alignment on the substrate 16. Should this part be the chip 14,it will be understood that the fibre 15 remains tightly gripped in theslot 17 during this operation since the temperatures of fibre and chipremain approximately equal.

[0055] While this thermal gripping may occur in both dimensions, it ispreferred that the pegs are gripped in a direction laterally of thefibre only. The fit in the other dimension may be accurate, butpreferably the holes are shaped so a degree of movement along the fibredirection is accommodated before thermal equilibration, so that, forexample, the end of the fibre may be precisely and tightly abuttedagainst the optical component or guide on the substrate 16.

[0056] The pegs and holes are sized so that when the pegs are coldrelative to the holes, the pegs are capable of entering the holes, butwhen there is no temperature differential therebetween, the pegs aresufficiently large to exert a force on the walls of the holes therebypreventing removal.

[0057] Thus in this modification it is preferred to assemble the chip 14with the fibre 15 mounted therein by cooling the fibre and placing itinto position. After temperature equilibration sufficient for the fibreto be gripped, the assembly is cooled and/or the substrate 16 is heatedthe chip 14 is inverted, and the pegs 18 are inserted into the holes 19.After any necessary adjustment of position of the chip 14, thetemperatures of the chip 14 and substrate 16 are allowed to equilibrateso that the chip is firmly mounted on the substrate.

[0058] Where it is required to couple the fibre 7 to a component such asa waveguide at or below the surface 26 of the substrate 16, an area ofthe substrate surface somewhat larger than that corresponding to thecross-section of chip 10 may be removed to the appropriate depth tobring the fibre end immediately adjacent the waveguide or othercomponent with their optical axes aligned. This is shown by thedepression 31 in the substrate surface 26 in FIGS. 5 and 6, and as canbe seen in FIG. 5 the fibre core is well below the level of surface 26,at a location suitable for alignment with a waveguide located below thesurface 26. In this type of construction, locating pins may be sitedelsewhere, for example as shown there are pins 30 projecting from theside of the chip which can slide down into corresponding slot aperturesin the substrate 16. The pins 30 can be a loose fit for fine adjustmentof the position of the chip 14, an accurate fit for precise location inthe direction of the fibre, or (optionally) they may be sized to begripped in the apertures by the thermal process outlined above (orretention may be by any other known means, such as adhesive). Also shownin FIG. 6 is the feature that the chip may be mounted so that its edgecoincides with (or optionally extends beyond) the edge of substrate 16,whereas FIG. 3 shows the case where it stops short of the edge. Theupper surface of chip 14 may lie flush with or below the surface 26 asrequired.

[0059] By making the depression 31 with vertical side walls which areappropriately spaced, it is also possible to use these walls to grip thesides of the chip 14 by the thermal process generally described above.That is to say, the assembly of chip and fibre is cooled so that it canenter between the side walls with the fibre end 22 abutting and alignedwith the component 24, and then allowed to warm so that it expands andexerts a retaining force on the walls. In such a case, the pins 30 couldbe omitted. Where the depression 31 occurs entirely within the boundaryof surface 26 rather than being located at one edge as shown, it ispossible to arrange for both opposed pairs of side walls to effect suchgripping. However, this is not preferred on account of the additionalstress which may be exerted on the substrate 16.

[0060] However, in many cases it is preferable to locate and grip thechip between one pair of opposed vertical walls and to allow relativemovement between the chip and substrate along the other dimension priorto equilibration. For example by locating the chip between the pair ofwalls parallel to the fibre axis, it remains possible to slide thecooled chip in the direction of the fibre axis to obtain a good opticalinterface with a buried waveguide etc., which is maintained aftertemperature equilibration by the gripping effect of the walls parallelto the fibre.

[0061] The embodiments of FIG. 3 to 5 may be modified so that two ormore chip substrates are mounted on or in the substrate 16, eithersimultaneously or sequentially. The fibres (or other optical components,see below) in the various chip substrates may be optical alignment, andpreferably in abutment. Also, the slot in at least one chip substratemay comprise more than one optical component.

[0062] The embodiment of FIG. 7 shows a silica optical fibre 3 which isretained within the groove 2 of a silicon substrate 1 by wall shaping.The protective coating of a commercial fibre is stripped to reveal thesilica cladding, of nominal diameter 125 microns, and it is insertedinto the slot 2, which has been dry-etched to a depth of 130 microns (orat least greater than the fibre radius) and a width of 126 microns. Theassembly is thermally oxidised at an elevated temperature, typically 800to 900° C. for 18 hours, but less than the glass transition temperatureof the fibre. The resulting conversion of the material of the walls andsubstrate surface to silicon dioxide 33 results in an increase in volumeand a reduction in the slot width. As shown, the silicon dioxide 33eventually thickens to such an extent that it grows round the fibre 3for example in regions 34 (wall shaping). It may also exert a mechanicalforce on the fibre for example at pinch (compression) points 35 and alower point 36, although the oxidation tends to be self-limiting, sothat excessive mechanical forces are avoided. Subsequent cooling of theassembly after oxidation tends to increase the mechanical retention ofthe fibre in the substrate by differential thermal shrinkage. As shown,regions can still exist in the groove 2 which are not filled with oxide.

[0063] This technique is useful not only for silicon/silica basedwaveguides as shown, but also for silicon/silica based hollowwaveguides, silicon/silica based micro-opto-electromechanicalstructures, and silicon/silica based vertical cavity structures, forexample.

[0064] The thermal oxidation process is preferably a “wet” process. Thetime could be reduced by initially porosifying the sidewalls usinganodic etching for example, to make a larger area available foroxidation. It will be understood that this process may be used asappropriate in lieu of, or in addition to, the differential processmentioned in relation to the earlier embodiments.

[0065] The above particular description has generally been in terms ofthe mounting and gripping of an optic fibre or fibres. While circularcross-section fibres are illustrated, other cross-sections of fibre maybe similarly accommodated. Furthermore, it should be understood thatwhile the invention is particularly suitable for use with fibres, atleast one fibre may be replaced by another (small) optical component. Inparticular, such a component could be a fibre laser or other lightsource, or a photodetector. The invention thus makes it possible, forexample, to form a system in which light is efficiently coupled betweena source or detector and a fibre, or between a source and detector via afibre.

1. An optical arrangement comprising a first rigid substrate having aslot and a first optical element mounted within the slot, said elementbeing firmly secured in place solely by mechanically contact with thesides of the slot.
 2. An optical arrangement according to claim 1wherein the substrate is of a ceramic or crystalline material.
 3. Anoptical arrangement according to claim 1 wherein said slot hassubstantially parallel sides, and said element is mechanically retainedunder compression by the sides of the slot.
 4. An arrangement accordingto claim 1 wherein said element is mechanically retained by the shapingof the sides of the slot.
 5. An arrangement according to claim 1 whereinsaid element is non-releasably mechanically retained by the sides of theslot.
 6. An arrangement according to claim 1 wherein the element is insubstantial optical alignment with an optical component within thesubstrate.
 7. An arrangement according to claim 6 wherein the element isin substantial abutment with said optical component.
 8. An arrangementaccording to claim 6 wherein said component is formed within the firstsubstrate.
 9. An arrangement according to claim 8 wherein said componentis waveguide.
 10. An arrangement according to claim 6 wherein saidcomponent is a second optical element which is mechanically retained bythe sides of the slot.
 11. An arrangement according to claim 10 whereinsaid second element is an optic fibre.
 12. An arrangement according toclaim 1 wherein said slot commences at one edge of the first substrate.13. An arrangement according to claim 1 wherein said slot extendsbetween opposed edges of the first substrate.
 14. An arrangementaccording to claim 1 wherein said first element is an optic fibre. 15.An optical device comprising an arrangement according to claim 1 whereinsaid first element is an optic fibre, and wherein said first substrateis mounted to a second substrate.
 16. An optical device according toclaim 15 wherein a surface of said second substrate includes an opticalcomponent mounted on formed thereon, and said first substrate is mountedto the said surface of said second substrate with the optic fibre inalignment with said optical component.
 17. An optical device accordingto claim 15 wherein a surface of said second substrate includes anoptical component recessed or formed therein, and said first substrateis mounted in a depression in the said surface of said second substratewith the optic fibre in alignment with said optical component.
 18. Anoptical device according to claim 15 wherein the first and secondsubstrate have co-operating projections and holes or slots for relativealignment thereof.
 19. An optical device according to claim 18 whereinat least one projection is a slightly oversize fit in its hole or slotin at least one dimension of the substrate surface, such that it may becooled relative thereto for insertion, and is gripped thereby ontemperature equalisation.
 20. An optical device according to claim 18 orclaim 19 wherein at least one projection is an accurate fit in its holein at least one dimension of the substrate surface for precise locationin said at least one dimension.
 21. An optical device according to claim18 wherein all projections are sized relative to the holes or slots sothat there is latitude for adjustment of alignment in one or bothdimensions of the second substrate surface.
 22. An optical deviceaccording to claim 15 wherein said depression has vertical opposed wallsbetween which said first substrate is gripped mechanically undercompression.
 23. An optical device according to claim 15 wherein an endof the optic fibre lies substantially flush with an edge of the secondsubstrate.
 24. An optical device according to claim 15 wherein the fibrehas a lateral portion projecting above the surface of the firstsubstrate, and the second substrate is recessed to accommodate theprojecting portion.
 25. A method for mounting an optical element in asubstrate by forming a parallel sided slot in the substrate, thedifference between a dimension of the optical element and the slot widthat ambient temperature being a very small positive amount, producing apositive temperature differential between the substrate in the region ofthe slot and the optical element such that said dimension is now no morethan the slot width, inserting the optical element therein andthereafter allowing the temperatures of the optical element andsubstrate to equilibrate.
 26. A method according to claim 25 wherein thestep of producing a positive temperature differential includes coolingthe optical element.
 27. A method according to claim 25 wherein the stepof producing a positive temperature differential includes heating thefirst substrate.
 28. A method according to claim 25 and comprising thefurther step of bonding the optical element to the first substrate. 29.A method according to claim 28 wherein the bonding step comprisesapplying adhesive to the optical element and first substrate.
 30. Amethod according to claim 25 and comprising the further step ofthermally oxidizing one or both of the first substrate and the opticalelement in the region of contact.
 31. A method of mounting an opticalelement in a substrate by forming a parallel sided slot in thesubstrate, inserting the optical element therein so that it is incontact with or immediately adjacent the walls of the slot, andthereafter oxidising the material one or both of the first substrate andthe optical element in the region of contact whereby to increase thedimension of the oxidised material so to mechanically retain the elementwithin the slot.
 32. A method according to claim 31 wherein the opticalelement is an optic fibre and said dimension is the or a diameter of thefibre, the slot depth being greater than half said dimension.
 33. Amethod according to claim 31 wherein the depth of the slot is less thanthe said dimension so that a portion thereof projects above the surfaceof the first substrate.
 34. A method according to claim 31 wherein thedepth of the slot is equal to or greater than said dimension.
 35. Amethod according to claim 31 wherein the substrate includes or isprovided with an optical component with an input surface or aperature,and the inserting step comprises butting an end of the optic fibreagainst said aperture.
 36. A method according to claim 35 wherein theslot forming step is controlled so that the fibre contacts the bottom ofthe slot when in alignment with said aperature.
 37. A method accordingto claim 35 wherein said component is formed integrally within thesubstrate, adjacent or spaced below its surface.
 38. A method accordingto claim 35 wherein said component is a separate component mountedwithin the substrate.
 39. A method according to claim 38 wherein saidcomponent is mounted within a section of said slot.
 40. A methodaccording to claim 39 wherein said component is mounted within saidsection by producing a positive temperature differential between theslot section and the component, inserting the component, and allowingthe temperature differential to disappear.
 41. A method according toclaim 38 wherein said component is another optic fibre.
 42. A methodaccording to claim 38 wherein said fibre and said component are mountedsimultaneously to said substrate.
 43. A method according to claim 38wherein said fibre and said component are mounted sequentially to saidsubstrate.
 44. A method according to claim 32 and comprising the furtherstep of mounting the first substrate to a second substrate with thefibre in optical alignment with an optical component on or in the secondsubstrate.
 45. A method according to claim 33 wherein a groove is formedin the second substrate to house the projecting portion of the fibre.46. A method according to claim 44 wherein the first substrate ismounted so that the fibre abuts the said component.
 47. A method ofcoupling two optic fibres comprising the steps of forming a parallelvertical sided slot in the surface of a first substrate, the differencebetween the diameter of the fibres and the slot width at ambienttemperature being a very small positive amount, producing a positivetemperature differential between the substrate in the region of the slotand the optic fibres such that the fibre diameter is now no more thanthe slot width, inserting the optic fibres therein with their endsbutting, and allowing the said temperature differential to disappear.48-51. (Cancelled)
 52. A method according to claim 25 wherein thesubstrate is rigid.
 53. A method according to claim 52 wherein thesubstrate is of a ceramic or crystalline material.
 54. A methodaccording to claim 31 wherein the substrate is rigid.
 55. A methodaccording to claim 54 wherein the substrate is of a ceramic orcrystalline material.
 56. A method according to claim 47 wherein thesubstrate is rigid.
 57. A method according to claim 56 wherein thesubstrate is of a ceramic or crystalline material.
 58. A methodaccording to claim 25 wherein the optical element is an optic fibre andsaid dimension is the or a diameter of the fibre, the slot depth beinggreater than half said dimension.
 59. A method according to claim 25wherein the depth of the slot is less than the said dimension so that aportion thereof projects above the surface of the first substrate.
 60. Amethod according to claim 25 wherein the depth of the slot is equal toor greater than said dimension.
 61. A method according to claim 25wherein the substrate includes or is provided with an optical componentwith an input surface or aperature, and the inserting step comprisesbutting an end of the optic fibre against said aperture.